It is well known that aircraft engines operate at substantially high temperatures and a portion of the air ingested by the engine is utilized for cooling, among other components, the stator structure in the hot sections of the engine. Typically, compressor bleed air is diverted to be routed to the outer air seals that surround the tips of the turbine blades and then discharged into the gas path of the engine or elsewhere. It is conventional to design the engine with the requisite amount of air to maintain the components within their structural integrity for the most severe conditions encountered. This establishes the amount of air bled from the compressor over the entire flight envelope. Obviously, at certain engine operating modes, more air than is actually necessary will be bled from the compressor hence incurring a penalty in engine performance.
It is also desirable to maintain the gap between the tips of the turbine blades, particularly the axial flow type, and its outer air seal as small as possible throughout the flight envelope. As will be appreciated, since the material of the stator components and the turbine rotor is different, and since the inertia effect has an influence on the growth of the rotor, the stator components, i.e., the engine case, outer air seal and support mechanism, grow at a different rate than the growth of the rotor. When the power is relaxed, the casing will tend to contract to a smaller diameter, as will the rotor, due to lower temperature and inertia leaving a gap between the tips of the blades and outer air seal. It is apparent that the transient conditions will establish the necessary gap for the steady state condition. This gap, however, is a leakage path for the engine's working medium being directed into the turbine airfoils for doing useful work. The amount of leakage of this working medium likewise results in a penalty to the engine's performance. There are other flight conditions that present similar problems as just described and can be more aggravated. Thus, for example, bodies, where the power lever is chopped (decelerate engine) and immediately re-burst (accelerate engine), can be a significant challenge to the engine designer because the components, i.e. rotor and stator, are already in a heated condition prior to a transient.
The industry has seen a number of different schemes for meeting this challenge. For example, U.S. Pat. No. 4,069,662 granted to I. H. Redinger, Jr. et al on Jan. 24, 1978 and assigned to United Technologies Corporation, the same assignee as this patent application, is a system that impinges cool air on the engine case at a selected engine operating condition in order to contract the case, and force the outer air seals which are tied to the case toward the tips of the blade to reduce the gap. However, systems that may be satisfactory for engines used to power commercial aircraft may not necessarily be satisfactory for engines used in military applications.
Thus, the particular application and engine mandate different techniques to meet this challenge not only to arrive at systems that will reduce the gap, but also to arrive at solutions that seek the least amount of leakage possible, if any.
We have found that we can attain an improvement in engine performance of a fan-jet engine by combining the control of the cooling air for the turbine's outer air seal and control of turbine blade tip clearance. This method contemplates throttling the cooling air to the outer air seal at preselected part power conditions of engine operations and allowing the air from the fan to scrub the outer engine case to cool and shrink it to force the outer air seals tied to the engine case to reduce the gap between it and the tips of the turbine blades. In this configuration, two supply routes are utilized, where one continuously supplies cooling air from the compressor, to the outer air seals and another throttles the air from that same source at predetermined conditions of engine operation. The continuous supplied air is routed into the outer air seal in such a manner as to avoid scrubbing the inner wall of the turbine case. The throttled cooling air is routed to intentionally scrub the inner walls of the turbine case. The reason being is that when the throttled cooling air is turned off, the much cooler air discharging from the fan that scrubs the outer wall of the engine case enhances the cooling effect and attains a tighter blade tip clearance.